Change in shape after adding mirror modifier
The reason for the changed shaped is the Merge option.
The vertices of the blade are directly on the merge plane XY but they are connect to vertices on the handle, which don't get merged. This result in some different connections.
A solution could be to turn off the merge option on the mirror modifier.


BUT...
... your topology is horrible. Here are some steps to fix it.

Grab the faces of the blade and move them down, of the XY merge plane. ⇧ Shift RMB &gt; GZ

Don't use triangles, remove the upper edge of the triangle at the transition. ⎈ Ctrl↹ Tab &gt; E,  LMB &gt; XECreate a new vertice by extruding.  LMB, E &gt; [drag] &gt;  RMB

Turn on the mirror modifier again (not subsurf though), select the edges along the ridge and extrude them upwards so the mirrored parts are joined.

Your topology should be a clear flow of quads. If you wish you can counteract the smoothing of the subsurf by marking certain edges as sharp. ⇧ Shift LMB select them and mark sharp ⇧ ShiftE.